Annealed Steel Microstructure Refinement for Die-Casting
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Solution Overview
Problem
Conventional methods fail to consistently prevent the formation of coarse austenitic grains at quenching temperature in die-casting dies, leading to reduced impact value and early cracking, especially in large-sized dies.
Innovation Solution
An annealed steel material with a specific composition and multiple annealing treatments above and around the Ac3 transformation point to refine the microstructure, ensuring the largest ferritic grain size is 120 μm or less and the area ratio of carbides is between 3.0% and 10.5%, thereby suppressing the generation of coarse austenitic grains.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If heating temperature is increased and holding time is extended to achieve complete austenitic transformation, then transformation completeness is improved, but austenitic grain size increases leading to coarse grains
Solution Approach 1:
The invention optimizes the heating temperature range to 850-950°C and controls holding time to prevent austenitic grain coarsening while ensuring complete transformation. This parameter optimization resolves the contradiction by finding the optimal window that achieves both transformation completeness and grain size control.
Solution Approach 2:
The invention performs preliminary austenitic transformation during the heating phase before quenching, ensuring that the transformation is substantially complete before the quenching process begins. This preliminary action prevents the need for excessive heating that would cause grain coarsening.
2Stability of the object's composition
If heating temperature is increased to ensure complete austenitic transformation, then transformation completeness is improved, but generation of coarse austenitic grains increases
Solution Approach 1:
The invention changes the heating temperature parameter to a controlled range of 850-950°C, which is sufficient to achieve complete austenitic transformation without causing excessive grain growth. This parameter change eliminates the harmful effect of coarse grain generation while maintaining transformation completeness.
Solution Approach 2:
The invention rapidly progresses through the heating phase to complete austenitic transformation quickly within the optimized temperature range, then immediately proceeds to quenching. This rushing through the critical phase prevents prolonged exposure to temperatures that would cause grain coarsening.
3Stability of the object's composition
If holding time is extended to achieve complete austenitic transformation, then transformation completeness is improved, but austenitic grain size increases
Solution Approach 1:
The invention optimizes the holding time parameter within the controlled heating temperature range to achieve complete transformation without excessive grain growth. By coordinating holding time with the optimized temperature range, the invention resolves the contradiction between transformation completeness and grain size control.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively stabilizes a fine-grained microstructure at quenching temperature, enhancing the impact value and preventing early cracking in die-casting dies, even for large cross-sectional sizes.
Implementation Method 1
heating the steel material to a temperature exceeding [Ac3 transformation point −20° C.] and [Ac3 transformation point +60° C.] or lower
Implementation Method 2
performing a plural times of an annealing treatment on a steel material in which the annealing treatment contains heating the steel material
Data Source
AI summary
The present invention provides an annealed steel material having a composition containing, in mass %, 0.28≤C≤0.42, 0.01≤Si≤1.50, 0.20≤Mn≤1.20, 4.80≤Cr≤6.00, 0.80≤Mo≤3.20, 0.40≤V≤1.20, and 0.002≤N≤0.080, with the balance being Fe and unavoidable impurities; in which the annealed steel material has a cross-sectional size of a thickness of 200 mm or more and a width of 250 mm or more, and a hardness of 100 HRB or less; and in which a diameter of a largest ferritic grain observed in a microstructure is 120 μm or less in terms of a perfect circle equivalent, an area ratio of carbides is 3.0% or more and less than 10.5%, and an average particle diameter of the carbides is 0.18 μm or more and 0.29 μm or less.